bootconfig: Split parse-tree part from xbc_init

Split bootconfig data parser to build tree code from
xbc_init(). This is an internal cosmetic change.

Link: https://lkml.kernel.org/r/163187296647.2366983.15590065167920474865.stgit@devnote2



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>